Confused why is there so much tension between me and my boyfriend brother?

Hey there don't know quite why am asking this but here goes. I have been with my boyfriend for almost 3 years now but the problem is his brother. Were not close or anything and I am convinced he hates me. Every time me and his brother are alone we will talk to each other and get on but I will feel awkward. And then when other people are about we won't even make eye contact or look at each other. I feel shy around him but at the same time feel like we could talk for hours. Every time we talk we give each other cheeky smiles but why is there so much tension I thought maybe he just doesn't like me but his new girlfriend told me he talks about me and how nice I am and honestly I think he a nice guy But am so fucking confused why do we act awkward around each other